Those trillions of forex $ trades are mostly just gambling, not transactions that ever directly escape into the real world. Since oil has been almost exclusively traded in $ it behoves countries, corporations, etc to have a supply of $ to buy the stuff that they know they will need. Holding a supply of $ effectively provides a hedge against currency fluctuations adversely affecting the cost of oil (it has been noticeable that the $ price of oil has been very insensitive to $ fluctuations versus other currencies the last 3 years).

The US$ has been the predominant holding in central banks' reserves these last 20 or 30 years, there were few alternatives. Some countries would hold smallish amounts in currencies of countries they did a lot of trade with but otherwise, apart from a bit of gold, the reserves were just about all US$. Now there are alternatives: the Euro is seen as credible and stable and much trade is done with Euroland, gold is becoming seen as another diversification since inflation concerns are rising. Additionally there is growing concern that US deficits are not sustainable over the medium to long term hence the $ is likely to decline.

So, the IOB, while not directly having a significant impact on the $ or US economy, is likely to have an effect. I see that as being twofold. First, it is a symptom of concern over the US$ as de facto global reserve currency, and of the US economy and its unsustainable deficits. Second, it is an opportunity / encouragement / excuse for a diversification from US$ reserves and US$ denominated assets.

That effect is likely to be slow. One does not wish to crash the $ while one has $ assets one wishes to convert, diversification will be a steady trickle - unless events force otherwise.

The big risk and question is how the US administration thinks. If it is determined to try to maintain the $ hegemony at all costs it may decide to intervene militarily in Iran. Personally I think that is insane and any who seriously argue for it (on those grounds) or do it, should be immediately deposed and put away for treatment. I don't honestly believe that GW is quite that insane.

Ultimately the US$ must decline, central banks will diversify away from the $. It is pointless attempting to fight that process, only worthwhile managing it the best one can (and it has been managed well so far). The best option for the US is to increase its energy efficiency, reduce its dependence on imported energy and develop its energy and transportation infrastructure appropriately. I see minimal signs of that from this US administration, though several states have started to make progress.